Commercial vehicles such as fire truck pumpers, sewer cleaners, and oil field pumpers include an auxiliary gearbox or transfer case that is connected to the output shaft of the vehicle's transmission. In auxiliary equipment mode, the transfer case drives auxiliary equipment, and the transmission controller provides a secondary mode of operation. A secondary mode of operation could be a single forward range or all ranges with a shift schedule specific to the auxiliary equipment.
The process of transfer case engagement and disengagement with auxiliary equipment takes place with the transmission output shaft locked by applying locking clutches. The locking clutch application is referred to as Neutral Very Low (NVL) range. In NVL, auxiliary equipment engagement and disengagement is difficult to achieve when gear teeth do not line up properly such that a “gear-clash” condition exists. “Gear-clash” in the transfer case cannot be overcome with a transfer case actuator. Operators will attempt to eliminate “gear-clash” by selecting drive or reverse. A high failure rate of transfer case actuators is attributed to “gear-clash”. Another “gear-clash” condition exists when the gear teeth have been lined up properly, however, the gear teeth do not slide to full engagement due to transmission drag torque on the gear teeth.
Therefore, there is a need for a system and method to preventing “gear-clash” that addresses both gear teeth alignment and gear teeth engagement. The system and method for preventing “gear-clash” should operate during engagement and disengaging of the auxiliary equipment.